Predictions of the long term stability of major ice sheets depend on the ability to identify where melt is occurring and to monitor changes over time. Particularly important is the detection of melting along the grounding line, where a glacier contacts solid earth, as this melting significantly increases the speed at which glaciers flow, increasing the risk of ice sheet collapse. Passive seismology offers an effective, relatively low cost and precise means of measuring the dynamic changes in the ice by enabling continuous monitoring of the critical contact between the ice sheet and the subsurface.
Our study's objectives were to 1) identify the attenuation characteristics of glacial ice and the contrast within fractured and fluid-filled zones, 2) map fluid–solid contacts within the ice sheets, particularly between ice and the underlying solid earth, 3) identify changes in the system over time, and 4) identify an optimal network configuration for the purposes of monitoring critical points within the glacier. We determined that the attenuation of seismic energy is highly sensitive to the presence of fractures and water within glacial ice. The ratio of attenuation of P wave energy to S wave energy (Qp/Qs) in particular is highly sensitive to porous regions where water is free to flow. Qp/Qs is typically greater than one (>1) in hard, rock-like portions of the glacier but drops below one (<1) where fractures are common. We observed significant variation along the Beardmore glacier, with high values beneath the solid “blue ice” and much lower values within the dynamic glacier. Upstream, where the glacier is grounded, attenuation appears lowest in large crevasse-like structures, a few of which extend to the base of the glacier, suggesting regions where the glacier is most susceptible to sliding. At the terminus, where the glacier floats free and is subject to tidal forcing, we see evidence of significant fracturing extending throughout the glacial column. Our approach works even when applied to passive seismic data.
The project advances Lawrence Livermore National Laboratory's initiative for climate and energy security and leverages the Laboratory's core competency in earth science to yield strategic impacts in energy security, environmental quality, and economic growth.
Matzel, E. 2020. "Using Seismic Interferometry to Identify and Monitor Fluids in Geothermal Systems." Proceedings World Geothermal Congress 2020. LLNL-CONF-785200.
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